The liquid direct-feed fuel cell is a device that generates electrical energy from the oxidation of organic fuels. Jet Propulsion Laboratory "JPL" developed a liquid direct-feed fuel cell using a solid-state electrolyte, preferably a solid polymer cation exchange electrolyte membrane. The subject matter of this implementation is described in U.S. Pat. No. 5,599,638, U.S. patent application Ser. No. 08/569,452 (Patent Pending), and U.S. patent application Ser. No. 08/827,319 (Patent Pending) the disclosures of which are incorporated by reference to the extent necessary for proper understanding.
FIG. 1 illustrates a typical structure of a JPL fuel cell with an anode 110, a solid electrolyte membrane 120, and a cathode 130. An anode 110 is formed on a first surface 140 of the solid electrolyte membrane 120 with a first catalyst for electro-oxidation. A cathode 130 is formed on a second surface 145 thereof opposing the first surface 140 with a second catalyst for electro-reduction. The anode 110, the solid electrolyte membrane 120, and the cathode 130 are hot press bonded to form a single multi-layer composite structure, referred to herein as a membrane electrode assembly "MEA" 150. An electrical load 160 is connected to the anode 110 and the cathode 130 for electrical power output.
A fuel pump 170 is provided for pumping an organic fuel and water solution into an anode chamber 180. The organic fuel and water mixture is withdrawn through an outlet port 190 and is re-circulated. Carbon dioxide formed in the anode chamber 180 is vented through a port 1100 within tank 1120. An oxygen or air compressor 1130 is provided to feed oxygen or air into a cathode chamber 1140.
Prior to use, the anode chamber 180 is filled with the organic fuel and water mixture. The cathode chamber 1140 is filled with air or oxygen either at ambient pressure or in a pressurized state. During operation, the organic fuel in the anode chamber 180 is circulated past the anode 110. Oxygen or air is pumped into the cathode chamber 1140 and circulated past the cathode 130. When an electrical load 160 is connected between the anode 110 and the cathode 130, electro-oxidation of the organic fuel occurs at the anode 110 and electro-reduction of oxygen occurs at the cathode 130. Electrons generated by electro-oxidation at the anode 110 are conducted through the external load 160 and are captured at the cathode 130. Hydrogen ions or protons generated at the anode 110 are transported directly across the solid electrolyte membrane 120 to the cathode 130. Thus, a flow of current is sustained by a flow of ions through the cell and electrons through the external load 160.
During operation, a fuel and water mixture in the concentration range of 0.5-3.0 mole/liter is circulated past the anode 110 within anode chamber 180. Preferably, flow rates in the range of 10-500 ml/min are used. As the fuel and water mixture circulates past the anode 110, the following electro-chemical reaction, for an exemplary methanol cell, occurs releasing electrons: EQU Anode: CH.sub.3 OH+H.sub.2 O .fwdarw.CO.sub.2 +6H.sup.+ +6e.sup.- (1)
Carbon dioxide produced by the above reaction is withdrawn along with the fuel and water solution through outlet 190 and separated from the solution in a gas-liquid separator. The fuel and water solution is then re-circulated into the cell by pump 170.
Simultaneous with the electrochemical reaction described in equation (1) above, another electrochemical reaction involving the electro-reduction of oxygen, which captures electrons, occurs at the cathode 130 and is given by: EQU Cathode: O.sub.2 +4H.sup.+ +4e.sup.-.fwdarw.2H.sub.2 O (2)
The individual electrode reactions described by equations (1) and (2) result in an overall reaction for the exemplary methanol fuel cell given by: EQU Cell: CH.sub.3 OH+1.5O.sub.2.fwdarw.CO.sub.2 +2H.sub.2 O (3)
During operation of the fuel cell, methanol is consumed at the anode 110. In order to maintain steady operation of the fuel cell system, the methanol concentration in the anode compartment 180 should be maintained. The concentration of methanol in the compartment can be sensed so that an appropriate amount of methanol is metered. The rate in which methanol is added to the system should be related to the rate of depletion of methanol in the system. Therefore, an accurate measure of fuel concentration is desirable for a fuel cell system.